Archive for February, 2010

I ended up going with a series circuit just like Jason has in the VRX-1. I exchanged the 10uh roid with two 21uh molded inductors in series and physically laying side-by-side. I switched out the vari-cap with a 260pf vari-cap as this was what I had been testing with. I can tune with no issues from about 7.008 to 7.040 MHz. I haven’t started the interfacing to the NS-40 yet and it may end up being a case where a separate circuit is used…just don’t know yet.

I have some other ideas I want to try still…but wanted to get to a point where I could use the VRX-1 again and play, so this why I have installed this circuit first. If you were just to use a 40 to 44 uh choke, inductor, coil, etc, you won’t get the wide freq swing. It seems that the molded inductors when placed next to each other will interact and provide for some rather interesting results.

Why am I hung up on the use of a VXO instead of Hartly oscillator or DDS circuit? I can get a good freq swing and great stability using these modified VXO oscillators and parts count is only five or six…plus I can build it for less than a buck. Doug DeMaw, W1FB (SK) liked the idea of using a VXO verse other methods and now I see why. QRP is homebrew and simple methods for me. I like to be able build things up in a weekend and then play with it. I wish I had gotten more into the “KISS Homebrew” method sooner!!!

Finished initial tests for the 40 meter vxo/osc design this morning. Not done with all tests as I want to look at the circuit that George, G3RJV used for his Universal VXO. What I did complete this morning allows me to swing from just under 7.00 Mhz to 7.037 Mhz in a linear manner and still being able to maintain a stable signal source. I need to put it on a dedicated piece of PCB board and mount it on the VRX-1/NS-40 rig and interface it to each.

Due to interaction of two of the inductors, a bit of playing around with it will be necessary to get it exactly where I want it to be (freq swing range). I used two 7.040Mhz xtals, three molded inductors (2, 18 and 20Uh) and a 260pf poly variable cap. The 2 and 18uh inductors lie side-by-side to allow interaction for a larger and more uniform swing.

I hope to try winding some roids this evening to see if I get the same results. Will post schematic and additional info on my web site later.

While cleaning up some old doc’s and notes last night, I ran across my copy of the 1999 FDIM Proceedings. George Dobbs3RJV had did a presentation detailing four small QRP projects for an evening or weekend, to include a Universal VXO (page 61 – 64). Reading through that article turned on a huge light bulb for me explaining why (or at least acknowledged that it is a reality and practical possibility to attain large swings) I was seeing a large swing on my 80 meter VXO project.

First of all, I love how George writes and speaks…a Master in His Own Time! Anyway, George commented that DJ1ZB had wrote in one of the G-QRP Sprats that the in-series inductor with the variable cap and xtal in a VXO circuit can be improved by breaking up the inductor into two nearly equal value inductors to be laid side-by-side for interaction. This allows for interaction which in-turn produces freq swings well beyond the normal swing for a VXO. Nothing was said about using two xtals, which I did, so maybe there is a lot of promise in this endevour. I sit here watching yet another 5 to 8 inches of white rain coming down, so great day to snuff some VXO fumes and see how the following might be useful … or not.

1. Two xtals in parallel and series

2. Using molded inductors (so far that is what I have used), but then try wingding some roids to redo the test using ferrite/iron powder

3. Does the oscillator circuit, the use of a 2N2222A, or 2N4401, etc, makes any difference in the amount of freq swing

4. What is the drift, stability, and noise associated with a Super VXO

I have built up the VXO and buffer circuit used in the VRX-1 for experimenting with Wide Swing VXO’s. I hope to start in the next day or two playing with different configurations of L/C/Xtal to find the most widest swing with stable and repeatable results. George’s circuit looks interesting so might have to build that one also to see which is best.

Having completed my VRX-1 receiver and NS-40 transmitter I found that the rock bound version of frequency control a bit limiting. I started digging around and found several articles and schematics for VXO’s that could pull a xtal more than just a few Hertz. SP5DDJ came up with what he calls his PIXIE-3, using a series LC circuit to pull a 80 meter xtal tens of Kilohertz which I thought was incredible.

I’ve messed around with it on a breadboard and can now pull a 3.582 MHz xtal down to 3.500 (and below) using a 200uf molded inductor and a 10 to 20 uH variable cap. I need to put the circuit onto a dedicated piece of PCB for mounting to a transmitter or receiver. Once on a PCB, the final inductor can cap values will probably change as I have noticed a lot of interaction on the workbench.

I want to also start playing around with a 40 meter version to use with my VRX-1/NS-40 combo rig. This 40 meter project will be more involved as I need it to work from 7.125 to 7.000 MHz.